Other Brine Impurities

The solubility products of the hydroxides cannot be used to calculate solubilities at high pH. Bivalent positive metal ions tend to shift according to the equilibrium  

FIGURE 7.88. Solubility of iron, cobalt, and nickel as a function of pH. 

Wth increasing alkalinity, becomes less soluble and HMOJ more soluble. Eventu- 

The pH generally should be kept below 9 or 10 to minimize the solubilities of these metals. 

The numbered subsections that follow consider impurities not amenable to satisfactory control by the standard primary treatment process. We divide the impurities dealt with here into three groups  

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Modern data acquisition and evaluation help to optimise the plant under review within a short period of time, to eradicate faults in plant operation and to determine the best materials for the operation of the chlorine electrolysis plant being examined. In this way, inter-relationships are examined between the energy consumption and variables such as membrane types, anode and cathode coatings, temperature, pressure, and concentrations as well as plant shutdowns, brine impurities, materials of construction and manufacturers. It is conceivable that other inter-relationships will come to light that have so far not been considered. 

Brine impurities originate from various sources and their effects on cell performance differ. We can classify them into several groups cationic impurities such as Mg " " and Ca " ", anionic impurities such as sulfate and halides other than chloride, and nonionic impurities such as alumina and silica. The effects of the interactions of certain combinations of impurities must also be considered. 

Brine impurities can also affect the anode coating. Barium sulfate, for example, precipitates on or in the coating and can destroy its catalytic action. Other ions harmful to the anode coating include lead, manganese, and fluoride. 

Section 4.8 and its appendix discuss the action of iodine in more detail. It interacts with other elements to form other precipitates in either NaCl or KO service. These precipitates include barium paraperiodate, and so there may be a synergistic effect when both barium and iodine are present in the brine. Table 4.8.8 lists commercial brine specifications for some of the common membranes [202]. The allowable concentrations of barium and iodine may be related to each other or to operating current density. Table 4.8.9 lists the adverse effeets of various brine impurities. There are reports of physically distinct forms of Ba-I precipitates, with some very fine particles that form in regions away from the main current paths through the membrane [203]. These tend to have relatively little effect on membrane performance [204], and Section 4.8.S.3 also discusses the development of membranes with enhanced resistance to the effects of iodine. 

Calcium carbonate also can be decomposed into its components by calcination or into CO2 and CaCl2 by digestion with hydrochloric acid. The CO2 can be absorbed into caustic solution to produce carbonate for brine treatment. If calcium is used to suppress the solubility of sulfates in raw salt, the CaCh also can be recycled. The latter technique can be limited by the presence of other metallic impurities. 

Excessive brine impurities or other severe operating problems can adversely affect the voltage of the cell. 

The brine clean-up consists of skimming and settling steps to free the solution from oil, clays, and other impurities. Sulfuric acid is then added until a pH of <2.5 is reached ensuring iodine Hberation by oxidation, precipitation of the soluble barium contained in the brine, and recovery of the remaining iodine. 

Brine Preparation. Rock salt and solar salt (see Chemicals frombrine) can be used for preparing sodium chloride solution for electrolysis. These salts contain Ca, Mg, and other impurities that must be removed prior to electrolysis. Otherwise these impurities are deposited on electrodes and increase the energy requirements. The raw brine can be treated by addition of sodium carbonate and hydroxide to reduce calcium and magnesium levels to below 10 ppm. If further reduction in hardness is required, an ion-exchange resin can be used. A typical brine specification for the Huron chlorate ceU design is given in Table 6. 

In the decomposer, deionized water reacts with the amalgam, which becomes the anode to a short-circuited cathode. The caustic soda produced is stored or evaporated, if higher concentration is required. The hydrogen gas is cooled by refrigeration to remove water vapor and traces of mercury. Some of these techniques are employed in different facilities to maximize the production of chlorine, minimize the consumption of NaCl, and also to prevent the buildup of impurities such as sulfate in the brine.26 The production of pure chlorine gas and pure 50% sodium hydroxide with no need for further concentration of the dilute solution is the advantage that the mercury cell possesses over other cells. However, the cell consumes more energy and requires a very pure brine solution with least metal contaminants and above all requires more concern about mercury releases into the environment.4... 

In the brine system, iodide is mixed with raw salt. It precipitates in membranes in the electrolysis process causing the loss of current efficiency in the membrane. Synergy effects in combination with other impurities have been reported [7, 8]. 

This chapter has introduced the RNDS application in the removal of impurities from brine destined for chlor-alkali electrolysis. On top of this, however, RNDS has potential use in other markets, including water treatment. Chlorine Engineers will continue its innovative work to meet the various requests coming from the chlor-alkali industry. 

More commonly, salt is produced by the evaporation of natural brines which are formed by pumping water into salt wells and subsequently pumping the resulting brine to the surface. These brines usually contain sodium chloride as the main solute, together with relatively small quantities of other salts, such as sodium and calcium sulfates and potassium and magnesium chlorides. Upon evaporation of the water, the resulting solid therefore consists largely of sodium chloride, but if a purer product is desired, the brine must be suitably treated to remove at least the major portion of the impurities. 

Many substances exist as mixtures. A mixture is made up of two or more substances that are not chemically bonded together e.g. sand and salt brine, which is salt and water and other impurities or a saline solution, which is made up of water with sodium chloride salt dissolved in it. The amounts of the substances can vary in a mixture, unlike a compound which has the same fixed proportions of atoms in every molecule and therefore in the bulk substance. 

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